1. Field of the Invention
This invention relates to an object tracing device highly suited for a video recorder or the like.
2. Description of the Related Art
The video apparatuses such as a video camera, a camera-integrated video recorder, etc., have remarkably advanced during the recent years. Efforts to automate the focusing (AF), exposure adjusting (AE) and gain adjusting (AGC) devices, etc., are being advanced. Of these devices, the use of the automatic focusing device especially for the video apparatuses of the above-stated kind is rapidly becoming popular even in the field of consumer appliances.
There are various automatic focusing methods. The well-known methods include a so-called active type automatic focusing method wherein an object to be photographed is illuminated with infrared rays or the like and an in-focus point is detected on the principle of the trigonometric measurement by using a reflected light thus obtained from the object; and a so-called passive method wherein a high-frequency component is extracted from a video signal and the lens is driven to shift its position in the direction in which the amount of the high-frequency component increases. The latter method has of late come to be preferably employed, because: it permits detection of a focusing state through a video signal obtained from image sensing means without using any elements for illuminating and receiving infrared rays. Besides, no faulty action results from, for example, a distance to the object or from shooting through glass. The applicability of this method is thus unrestricted by photographing conditions.
With the passive method employed, the focusing action can be accomplished on the basis of the high-frequency component of a video signal which corresponds to the whole image sensing plane of the image sensing means. If the whole image sensing plane is to be used as a focus detecting area, a so-called perspective contention tends to arise between a main object and a background. To avoid this, therefore, a focus detecting area is set only within a part of the image sensing plane which is, for example, located in the central part of the image sensing plane.
The arrangement to set such a limited area, in the central part of the image sensing plane is not only advantageous for the AF action but also enables the AE and AGC actions to be adequately performed by adjusting an exposure or a signal gain with weight attached to the signal obtained from within the area thus set without being affected by the luminance of the background under a so-called back-light condition or the like.
However, the above-stated advantageous effect is attainable only when a main picture-taking object is located in the central part of the image sensing plane where the focus detecting area is set. In cases where the main object moves outside of the focus detecting area, the AF, AE and AGC actions are faultily performed as these action are likely to be performed for some other object instead of the main object.
An automatic tracing device has been proposed as a solution of this problem, for example, in Japanese Laid-Open Patent Application No. SHO 60-249477. The automatic tracing device is arranged as follows: the high-frequency component of a spatial frequency is detectable in a greater quantity from the main object part than the other part. In view of this, the focus detecting area is set always in a position where the amount of the high-frequency component of the video signal reaches a peak value within the image sensing plane. This arrangement of the device enables the focus detecting area to follow the movement of the main object. Therefore, the lens can be continuously focused on the object despite of any movement of the object that takes place within the image sensing plane. The same advantageous effect is attainable also for the AE and AGC actions.
In the above-stated prior art device, however, the position in which the high-frequency component of the video signal reaches a maximum value is regarded as a feature point of the main object. Therefore, if the maximum value of the high-frequency component corresponding to some other object, such as a background object, is detected by mistake, the device thereafter would continue to have the lens focused on the mistaken object. The device thus rather causes inconvenience.
Another device has been proposed by Japanese Laid-Open Patent Application No. SHO 60-249477. This device is arranged as follows: the focus detecting area is set in the central part of the image sensing plane when the peak value of the high-frequency component drops below a given level and an object tracing action is resumed when the peak value comes to exceed the given level. In accordance with this arrangement, however. when the lens comes from an out-of-focus state to an in-focus state, the object must be located in the central part of the image sensing plane. If not, the camera must be moved and set by the photographer to have the object in the central part of the image sensing plane, even while the lens is still in an out-of-focus state, before the lens is brought into the in-focus state. The photographer is thus required to perform an additional step of operation during the process of photographing. This is not only burdensome but also tends to cause a faulty action.